1. Field of the Invention
The invention relates to an arrangement of optical elements in a microlithographic projection exposure apparatus, a projection objective with such an arrangement, a projection exposure apparatus with such an arrangement or such a projection objective and a method for the production of microstructured components with such an apparatus.
2. Description of Related Art
Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers to a suitable substrate, which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a mask, is projected onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are also often referred to as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied to the wafer.
One of the essential aims in the development of projection exposure apparatuses used for production is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, and this generally has a favorable effect on the performance of the microstructured components produced with the aid of such systems.
The size of the structures, which can be defined, depends primarily on the resolution of the projection objective. Since the size of the structure, which can still be resolved with a projection objective, is proportional to the wavelength of the projection light, one way of improving the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present are in the deep ultraviolet (DUV) spectral range, namely 193 nm and 157 nm.
Another way of improving the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an intermediate space, which remains between a last lens on the image side of the projection objective and the photoresist. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion lenses, can achieve numerical apertures of more than 1, for example 1.3 or 1.4. The immersion, moreover, not only allows high numerical apertures and therefore improved resolution but also has a favorable effect on the depth of focus. The greater the depth of focus is, the less stringent are the requirements for exact axial positioning of the wafer in the image plane of the projection objective.